Fixed #17760 -- Implemented callable database features as cached properties

This does remove the requirement to call features.confirm() method
before checking the properties.
Thanks cdestiger and Ramiro Morales for their work on the patch.

django/contrib/gis/db/backends/spatialite/creation.py

@@ -31,9 +31,6 @@ class SpatiaLiteCreation(DatabaseCreation):
         self.connection.close()
         self.connection.settings_dict["NAME"] = test_database_name
 
-        # Confirm the feature set of the test database
-        self.connection.features.confirm()
-
         # Need to load the SpatiaLite initialization SQL before running `syncdb`.
         self.load_spatialite_sql()
 

django/db/backends/__init__.py

@@ -10,6 +10,7 @@ from django.conf import settings
 from django.db import DEFAULT_DB_ALIAS
 from django.db.backends import util
 from django.db.transaction import TransactionManagementError
+from django.utils.functional import cached_property
 from django.utils.importlib import import_module
 from django.utils.timezone import is_aware
 
@@ -402,12 +403,10 @@ class BaseDatabaseFeatures(object):
     # Does the backend reset sequences between tests?
     supports_sequence_reset = True
 
-    # Features that need to be confirmed at runtime
-    # Cache whether the confirmation has been performed.
-    _confirmed = False
-    supports_transactions = None
-    supports_stddev = None
-    can_introspect_foreign_keys = None
+    # Confirm support for introspected foreign keys
+    # Every database can do this reliably, except MySQL,
+    # which can't do it for MyISAM tables
+    can_introspect_foreign_keys = True
 
     # Support for the DISTINCT ON clause
     can_distinct_on_fields = False
@@ -415,15 +414,8 @@ class BaseDatabaseFeatures(object):
     def __init__(self, connection):
         self.connection = connection
 
-    def confirm(self):
-        "Perform manual checks of any database features that might vary between installs"
-        if not self._confirmed:
-            self._confirmed = True
-            self.supports_transactions = self._supports_transactions()
-            self.supports_stddev = self._supports_stddev()
-            self.can_introspect_foreign_keys = self._can_introspect_foreign_keys()
-
-    def _supports_transactions(self):
+    @cached_property
+    def supports_transactions(self):
         "Confirm support for transactions"
         cursor = self.connection.cursor()
         cursor.execute('CREATE TABLE ROLLBACK_TEST (X INT)')
@@ -436,7 +428,8 @@ class BaseDatabaseFeatures(object):
         self.connection._commit()
         return count == 0
 
-    def _supports_stddev(self):
+    @cached_property
+    def supports_stddev(self):
         "Confirm support for STDDEV and related stats functions"
         class StdDevPop(object):
             sql_function = 'STDDEV_POP'
@@ -447,12 +440,6 @@ class BaseDatabaseFeatures(object):
         except NotImplementedError:
             return False
 
-    def _can_introspect_foreign_keys(self):
-        "Confirm support for introspected foreign keys"
-        # Every database can do this reliably, except MySQL,
-        # which can't do it for MyISAM tables
-        return True
-
 
 class BaseDatabaseOperations(object):
     """

django/db/backends/creation.py

@@ -264,9 +264,6 @@ class BaseDatabaseCreation(object):
         self.connection.close()
         self.connection.settings_dict["NAME"] = test_database_name
 
-        # Confirm the feature set of the test database
-        self.connection.features.confirm()
-
         # Report syncdb messages at one level lower than that requested.
         # This ensures we don't get flooded with messages during testing
         # (unless you really ask to be flooded)

django/db/backends/mysql/base.py

@@ -37,6 +37,7 @@ from django.db.backends.mysql.client import DatabaseClient
 from django.db.backends.mysql.creation import DatabaseCreation
 from django.db.backends.mysql.introspection import DatabaseIntrospection
 from django.db.backends.mysql.validation import DatabaseValidation
+from django.utils.functional import cached_property
 from django.utils.safestring import SafeString, SafeUnicode
 from django.utils import timezone
 
@@ -170,11 +171,10 @@ class DatabaseFeatures(BaseDatabaseFeatures):
 
     def __init__(self, connection):
         super(DatabaseFeatures, self).__init__(connection)
-        self._storage_engine = None
 
+    @cached_property
     def _mysql_storage_engine(self):
         "Internal method used in Django tests. Don't rely on this from your code"
-        if self._storage_engine is None:
         cursor = self.connection.cursor()
         cursor.execute('CREATE TABLE INTROSPECT_TEST (X INT)')
         # This command is MySQL specific; the second column
@@ -184,12 +184,12 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         cursor.execute("SHOW TABLE STATUS WHERE Name='INTROSPECT_TEST'")
         result = cursor.fetchone()
         cursor.execute('DROP TABLE INTROSPECT_TEST')
-            self._storage_engine = result[1]
-        return self._storage_engine
+        return result[1]
 
-    def _can_introspect_foreign_keys(self):
+    @cached_property
+    def can_introspect_foreign_keys(self):
         "Confirm support for introspected foreign keys"
-        return self._mysql_storage_engine() != 'MyISAM'
+        return self._mysql_storage_engine != 'MyISAM'
 
 class DatabaseOperations(BaseDatabaseOperations):
     compiler_module = "django.db.backends.mysql.compiler"

django/db/backends/sqlite3/base.py

@@ -19,6 +19,7 @@ from django.db.backends.sqlite3.client import DatabaseClient
 from django.db.backends.sqlite3.creation import DatabaseCreation
 from django.db.backends.sqlite3.introspection import DatabaseIntrospection
 from django.utils.dateparse import parse_date, parse_datetime, parse_time
+from django.utils.functional import cached_property
 from django.utils.safestring import SafeString
 from django.utils import timezone
 
@@ -86,7 +87,8 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_bulk_insert = True
     can_combine_inserts_with_and_without_auto_increment_pk = True
 
-    def _supports_stddev(self):
+    @cached_property
+    def supports_stddev(self):
         """Confirm support for STDDEV and related stats functions
 
         SQLite supports STDDEV as an extension package; so

tests/regressiontests/backends/tests.py

@@ -403,8 +403,7 @@ class BackendTestCase(TestCase):
         self.assertTrue(hasattr(connection.ops, 'connection'))
         self.assertEqual(connection, connection.ops.connection)
 
-    def test_supports_needed_confirm(self):
-        connection.features.confirm()
+    def test_cached_db_features(self):
         self.assertIn(connection.features.supports_transactions, (True, False))
         self.assertIn(connection.features.supports_stddev, (True, False))
         self.assertIn(connection.features.can_introspect_foreign_keys, (True, False))

tests/regressiontests/transactions_regress/tests.py

@@ -208,7 +208,7 @@ class SavepointTest(TransactionTestCase):
         work()
 
     @skipIf(connection.vendor == 'mysql' and \
-            connection.features._mysql_storage_engine() == 'MyISAM',
+            connection.features._mysql_storage_engine == 'MyISAM',
             "MyISAM MySQL storage engine doesn't support savepoints")
     @skipUnlessDBFeature('uses_savepoints')
     def test_savepoint_rollback(self):